Methods of making optical fiber gratings have become increasingly important. As optical fiber communication systems have advanced to carry higher bit rate signals on a greater number of optical channels, they have required fiber gratings in increasing number and sophistication to direct, compensate, separate, join and process the optical channels.
Phase masks are very useful in the fabrication of optical fiber gratings. Gratings consist of a sequence of periodic or near-periodic perturbations in the index of refraction along a length of grating. These perturbations are advantageously formed by providing a length of fiber having a photosensitive core and exposing the core to laser light that varies in intensity along the fiber length. Variation in laser intensity produces a corresponding variation in the index of refraction along the core. An appropriate pattern of variation produces a grating.
While a sequence of closely spaced low intensity and high intensity regions can be produced by exposing the fiber through an amplitude (slit mask) or by exposing the fiber to an interference pattern of laser beams, such techniques require meticulous and time-consuming work to achieve desired results. It is particularly difficult to produce large numbers of sophisticated gratings by such techniques.
Rather than individually writing each grating, it is preferable to write a phase mask which, when exposed to laser light, will produce a desired pattern on the fiber. The phase mask need be written only once. With relatively simple controls it can be subsequently used to rapidly and reliably produce a large number of nearly identical gratings.
A phase mask is typically made by coating a transparent substrate with photoresist, scanning the photoresist with laser light in an appropriately modulated pattern, developing the photoresist, and etching the substrate.
Unfortunately it is difficult to make phase masks for fabricating a number of important gratings. Some gratings, such as superstructure gratings, require illumination wherein the exposure source is modulated in phase. While lasers used to expose the photoresist in making phase masks can be modulated in amplitude, it is very difficult to modulate them in phase. Other phase masks require very rapid modulation of exposure. While lasers can be modulated in amplitude, it is difficult to change the amplitude as rapidly as some designs require. Accordingly there is a need for an improved method of making phase masks.